1. Field of the Invention
The present invention concerns a DC output control and protection device for a programmable control system such as a programmable automatic controller.
2. Description of the Prior Art
It is known to protect DC outputs of a programmable automatic controller against short circuits by means of a protection circuit associated with each output channel. A circuit of this kind usually comprises a processor unit of which an input pin is connected to an overcurrent detector which is associated with the output channel. The processor unit compares the detected current level with a threshold and delivers a signal if the current exceeds the threshold; the resulting signal is filtered and then applied to resettable storage logic means, such as a flip-flop, the output of which is connected to an output pin of the protection circuit. The disabling signal available at this output pin is applied to the inverting input of an AND gate which also receives a control signal from a processor or from a control unit of the programmable automatic controller; the output of the AND gate is applied to a power switch which controls the opening and the closing of the output channel according to the control signal and the disabling signal or a reset instruction.
The protection circuit therefore requires two connecting pins to acquire the current information and to disable the output affected by a short circuit. However, it would be desirable to protect the output channels by means of an integrated circuit having a minimal number of connecting pins.
A particular object of the invention is to simplify the connection of a protection device for DC outputs in a programmable system such as a programmable automatic controller, more specifically by acquiring the overcurrent information relating to an output and disabling the latter by means of a common connection.